Exit Channel Dynamics Research Articles

Details of exit channel dynamics of the ozonolysis of catechol in condensed phase: Product channels and product energy partitioning

Direct dynamics simulations on the ozonolysis reaction of catechol is performed in a vacuum and in an N2 bath to study the various aspects of the exit channel dynamics of the reaction. The simulations were conducted at 20 and 324 kg/m3 densities, with 200 and 293 N2 molecules in the bath, respectively. It is a post-transition state (TS) direct dynamics simulation, where the TS of the catechol+O3 system is taken as the reactant. High combustion temperatures of 800, 1000, and 1500 K are considered for the reactant excitation. In contrast, the bath is taken at 300 K. CO, H2O, CO2, O2, and small carboxylic acid (SCA) are the major channels observed for this reaction. Explicit study of the product channels along with rate constants of all the product channels are obtained. The rate constants are in the order of ns−1. The partitioning of the product's internal energy into its various modes and how the energy transfer occurs in the formation of the products is an exciting aspect of the study. Also reported in this work is the branching ratio of the products and comparison with the experimentally reported data. The energy partitioning study reveals that the dissociation dynamics of the catechol-O3 TS is not statistical. In contrast, the bath dynamics, which correspond to the energy transfer from the TS to the bath, is statistical in nature.

Determination of the Temperature-Dependent OH− (H2O) + CH3I Rate Constant by Experiment and Simulation

Abstract Experimental and simulation studies of the OH – (H2O) + CH3I reaction give temperature dependent rate constants which are in excellent agreement. Though there are statistical uncertainties, there is an apparent small decrease in the rate constant as the temperature is increased from − 60 to 125 ℃, and for this temperature range the rate constant is ∼ 1.6 times smaller than that for the unsolvated reactants OH – + CH3I. Previous work [J. Phys. Chem. A 117 (2013) 14019] for the unsolvated reaction found that the S N 2 and proton transfer pathways, forming CH3OH + I – and CH2I – + H2O, have nearly equal probabilities. However, for the microsolvated OH – (H2O) + CH3I reaction the S N 2 pathways dominate. An important contributor to this effect is the stronger binding of H2O to the OH – reactant than to the proton transfer product CH2I – , increasing the barrier for the proton transfer pathway. The effect of microsolvation on the rate constant for the OH – (H2O)0,1 + CH3I reactions agrees with previous experimental studies for X – (H2O)0,1 + CH3Y reactions. The simulations show that there are important non-statistical attributes to the entrance- and exit-channel dynamics for the OH – (H2O) + CH3I reaction.

Exit Channel Dynamics in a Micro-Hydrated SN2 Reaction of the Hydroxyl Anion

We report on the reaction dynamics of the monosolvated SN2 reaction of cold OH(-)(H2O) with CH3I that have been studied using crossed beam ion imaging. Two SN2 reaction channels are possible for this reaction: Formation of unsolvated I(-) and of solvated I(-)(H2O) products. We find a strong preference for the formation of unsolvated I(-) reaction products with respect to the energetically favored reaction toward solvated I(-)(H2O). Angle differential cross section measurements reveal similar velocity and angular distributions for all solvated and parts of the unsolvated reaction products. We furthermore find that the contribution of these two products to the total product flux can be described by the same collision energy dependence. We interpret our findings in terms of a joint reaction mechanism in which a CH3OH(H2O)···I(-) complex is formed that decays into either solvated or unsolvated products. Quantum chemical calculation are used to support this assumption.

State-correlated DC slice imaging of formaldehyde photodissociation: roaming atoms and multichannel branching

High-resolution slice imaging methods allow for detection of single product quantum states with sufficient velocity resolution to infer the full correlated product state distribution of the undetected fragment. This is a level of detail not available in previous studies of formaldehyde photodissociation, and in this application it reveals startling new aspects of unimolecular decomposition. The CO rotational distributions from near ultraviolet dissociation of formaldehyde are bimodal, and the imaging experiments allow us to decompose these into two dynamically distinct components: the conventional molecular dissociation over a high exit barrier, and a novel ‘roaming atom’ reaction in which frustrated radical dissociation events lead to intramolecular H abstraction, bypassing the transition state entirely. In probing the dynamics of the conventional molecular dissociation over the barrier, we use the complete vH2-jCO correlation to model the exit channel dynamics in new detail. Furthermore, these state-correlated measurements provide insight into radical–radical reactions and the underlying dynamics and energy dependence of the roaming pathway.

A classical trajectory study of the photodissociation of T1 acetaldehyde: The transition from impulsive to statistical dynamics

Previous experimental and theoretical studies of the radical dissociation channel of T(1) acetaldehyde show conflicting behavior in the HCO and CH(3) product distributions. To resolve these conflicts, a full-dimensional potential-energy surface for the dissociation of CH(3)CHO into HCO and CH(3) fragments over the barrier on the T(1) surface is developed based on RO-CCSD(T)/cc-pVTZ(DZ) ab initio calculations. 20,000 classical trajectories are calculated on this surface at each of five initial excess energies, spanning the excitation energies used in previous experimental studies, and translational, vibrational, and rotational distributions of the radical products are determined. For excess energies near the dissociation threshold, both the HCO and CH(3) products are vibrationally cold; there is a small amount of HCO rotational excitation and little CH(3) rotational excitation, and the reaction energy is partitioned dominantly (>90% at threshold) into relative translational motion. Close to threshold the HCO and CH(3) rotational distributions are symmetrically shaped, resembling a Gaussian function, in agreement with observed experimental HCO rotational distributions. As the excess energy increases the calculated HCO and CH(3) rotational distributions are observed to change from a Gaussian shape at threshold to one more resembling a Boltzmann distribution, a behavior also seen by various experimental groups. Thus the distribution of energy in these rotational degrees of freedom is observed to change from nonstatistical to apparently statistical, as excess energy increases. As the energy above threshold increases all the internal and external degrees of freedom are observed to gain population at a similar rate, broadly consistent with equipartitioning of the available energy at the transition state. These observations generally support the practice of separating the reaction dynamics into two reservoirs: an impulsive reservoir, fed by the exit channel dynamics, and a statistical reservoir, supported by the random distribution of excess energy above the barrier. The HCO rotation, however, is favored by approximately a factor of 3 over the statistical prediction. Thus, at sufficiently high excess energies, although the HCO rotational distribution may be considered statistical, the partitioning of energy into HCO rotation is not.

Exit-channel dynamics in barrierless unimolecular reactions: Criteria of vibrational adiabaticity

Conversion of translational into vibrational energy during the last step of a unimolecular reaction is brought about by the curvature of the reaction path. The corresponding coupling is analyzed by an angle-action reaction path Hamiltonian (RPH). The accuracy of the vibrational adiabatic approximation is found to be completely independent of the shape of the potential energy Vs. Vibrations are adiabatic when two independent dimensionless parameters are small. The first one, denoted as sigma, controls the dynamic coupling. The physical significance of the condition sigma<<1 is that the amplitude of the vibrations normal to the reaction path should be much smaller than the radius of curvature of the reaction path. The second parameter, denoted as mu, governs the static coupling. It results from the dependence of the vibrational frequency omega on the reaction coordinate s. The higher omega, the lower its derivative with respect to s and, more unexpectedly, the higher the translational energy epsilon, the lower mu is. A criterion for locating a particular dividing surface in barrierless reactions is proposed. This surface separates two regions of space: one where energy flows freely, and one where energy conversion between translation and vibration is hindered by adiabatic invariance. The nature of the dynamical constraint that prevents the product translational energy distribution from being fully statistical can be identified by a maximum entropy analysis. The constraint is found to bear on the translational momentum ps, i.e., on the square root of the translational energy epsilon1/2. This can be understood by applying Jacobi's form of the least action principle to the vibrationally adiabatic RPH.

I*(2P1/2) and Cl*(2P1/2) Production from Chloroiodobenzenes in the Ultraviolet

The relative quantum yields of $I*(^2P_{1/2})$ and $Cl*(^2P_{1/2})$ production, $\phi*(I)$ and $\phi*(Cl)$, respectively, have been measured at four different ultraviolet excitation wavelengths, e.g., 222, 236, 266, and 280 nm in the photodissociation of o-, m-, and p-chloroiodobenzenes. The measured I* and Cl* quantum yields are, with some exceptions, higher than those obtained, respectively, from iodobenzene and chlorobenzene, at the same wavelengths. While at most wavelengths the major fraction of the iodine atoms is produced in the excited state, the opposite is true for the chlorine atoms. Both direct and indirect dissociation pathways are involved in the production of I* atoms whereas Cl* is produced only by indirect pathways since direct excitation of the $\sigma$*$(C-Cl) \leftarrow n(Cl)$ transition is not possible at these wavelengths. The halogen atom in the ortho position is found to be most effective in enhancing the yield of the other spin-orbit excited halogen atom in the photolysis. While the nature of the initial transition, the extent of intersystem crossing in the excited states and the exit channel effects need to be considered in interpreting the quantum yield results, some factors seem to be more effective in influencing the final outcome. Induced dipole-induced dipole and quadrupolequadrupole interactions between the two halogen atoms (I and Cl in this case) seem to play an important role in the exit channel dynamics. These electrostatic interactions facilitate the intersystem transfer in the excited state and subsequent production of the spin-orbit excited halogen atoms.

Exit channel dynamics in the ultraviolet photodissociation of the NO dimer: (NO)2→NO(A 2Σ+)+NO(X 2Π)

The correlated angular and product rotational state distributions obtained in the 221.67 nm photodissociation of (NO)2 yielding NO(A 2Σ+)+NO(X 2Π) have been examined in the molecular beam using the velocity map ion imaging technique. The translational energy and angular distributions of selected rotational states of NO(A 2Σ+) products in N=0, 5, 6 for which the maximum energies available to the NO(X 2Π) products are 202.5, 142.5, and 118.5 cm−1, respectively, have been measured. The recoil anisotropy parameter of the photofragments, βeff, is 1.2±0.1, less than that previously measured at 213 nm (1.36±0.05). The correlated product state distributions near dissociation threshold agree with the predictions of phase space theory. These experimental results, as well as those obtained previously at 213 nm, are compared to statistical calculations, including v⋅J correlations. Application of the β-ET correlation model to the 213 nm results indicates that [NO(A,N),NO(X,J)] pairs with high NO(X,J) rotational levels are produced preferentially via planar dissociation, in contrast to the statistical expectation of the v⋅J correlation, which reveals no preference for planar dissociation. A mechanism involving vibrational predissociation with restricted intramolecular vibrational energy redistribution can explain both the observed scalar and vector properties. Specifically, the low frequency torsional (out-of-plane) mode does not couple efficiently to the other modes, especially at higher excess energies when the dissociation is rapid. On the other hand, the long-range attraction between NO(A) and NO(X), which is revealed both in the photodissociation dynamics of the dimer and in the quenching of NO(A) by NO(X), encourages long-range mode couplings and can explain the largely statistical rotational state distributions observed near threshold. From images obtained near threshold, the bond energy of the NO dimer in the ground state is determined to be 710±10 cm−1, in good agreement with previous results.

Accuracy of the energy partitioning data obtained by classical trajectory calculations on potential energy surfaces constructed by interpolation: H2CO→H2 + CO as an example

Capability of the classical trajectory calculation on a potential energy surface (PES) constructed by interpolation in reproducing experimental energy partitioning data has been tested. The title reaction, a prototype polyatomic dissociation which has been heavily investigated over the years, has been taken as the example. The product energy partitioning data obtained by scaling the classical trajectory results on PESs constructed at the moderately high quantum chemical levels of Hartree–Fock (HF), second-order Mo/ller–Plesset (MP2), quadratic configuration interaction single double (QCISD), and B3LYP were nearly level-independent and reproduced the experimental data almost quantitatively. The overall scheme, which is systematic, may become an important quantitative tool for the study of the exit channel dynamics in favorable cases.

Predissociation of the Hydroxymethyl Radical in the 3pz Rydberg State: Formaldehyde + Hydrogen Atom Channel

The photodissociation of the hydroxymethyl radical to hydrogen atom and formaldehyde was investigated following excitation to the origin band of the Rydberg 2A"(3pz) state of jet cooled CH2OD and CD2OH. D and H products were detected by 2-color laser ionization. The peaks in the photofragment yield spectrum of the D product from CH2OD correspond to peaks in the absorption spectrum, and this establishes hydroxymethyl as the source of the observed D atoms. D atoms appear as major products from CH2OD, but the H photofragment yield is not discernible above the background. On the other hand, in the dissociation of CD2OH, only H atoms are produced. It is concluded that isomerization to the methoxy radical is not important. Time-of-flight distributions of the D atom from CH2OD were obtained under core sampling conditions. The product translational energy distribution derived from the data is broad, indicating that the formaldehyde cofragment is produced in the ground electronic state but with an internal energy distribution that extends to the thermochemical limit. The recoil anisotropy parameter was estimated by comparing the intensity of the D signals obtained with parallel and perpendicular polarization of the photolysis laser. The observed anisotropic angular distribution suggests that the dissociation is fast, as also indicated by the line width. The results can be rationalized by a mechanism that involves nonadiabatic transitions from the initially excited Rydberg state to the ground state, with the final crossing occurring in a region of the potential energy surface that leads to direct O−D(H) fission without isomerization. Exit channel dynamics is probably responsible for the high vibrational excitation of the formaldehyde fragment.

An ab initio quasi-classical direct dynamics investigation of the F+C2H4→C2H3F+H product energy distributions

A direct dynamics technique, using energies, forces and second derivatives calculated at the UHF/6–31G* level of theory, was used to investigate product energy distributions of the F+C2H4→C2H3F+H collision reaction. The shifting and broadening of the product translational energy distribution as the system moves from the exit-channel barrier to the products was studied. Since properties associated with the rupturing C···H bond are similar for the C2H5‡ and C2H4F‡ exit-channel barriers, and integration of the C2H5‡→C2H4+H reaction is approximately 2.5 times faster than the C2H4F‡→C2H3F+H reaction, trajectories of the former reaction were propagated to gain insight into the exit-channel dynamics. Ensemble averaged results for C2H5‡ dissociation are well described by a model based on isotropic exit-channel dynamics which assumes that the product relative translational distribution arises from the centrifugal potential and relative translational energy distributions at the exit-channel barrier plus the exit-channel potential release. The width of the product translational energy distribution is sensitive to overall rotational angular momentum and its partitioning between C2H4···H‡ orbital angular momentum and C2H4‡ rotational angular momentum. The simulated product translational energy distribution for the C2H4F‡→C2H3F+H reaction is broadened by relative translation–vibrational couplings in the exit-channel and is similar to the distribution used to fit crossed molecular beam data. Approximately 50% of the available energy is in product relative translation, which also agrees with experiment. RRKM calculations indicate that a second reaction mechanism, involving 1–2 hydrogen migration prior to C···H bond fission, does not significantly contribute to C2H3F+H product formation.

Scattering state spectroscopy of the reaction Mg*(3s3p 1P1)+CH4→MgH(v=0,1;N)+CH3

We report scattering state spectroscopic studies of the chemical quenching dynamics of Mg*(3p(1P)) by CH4. We have measured the final-state resolved action spectra for the MgH(v=1,N) reactive product channels, following excitation of the Mg*(3p)–CH4 transient bimolecular collision complex. As in earlier work on the ground vibrational state of the product, we have found a strong electronic orbital alignment effect: Reaction to the vibrationally excited product follows only on the attractive excited potential-energy surfaces in “Π-like” symmetry. For both MgH(v=0 and 1) product channels we have found that the rotational quantum state distribution is independent of laser excitation wavelength, indicating that the rotational energy partitioning is determined by exit channel dynamics. However, our results show that the product vibrational energy disposal is a function of excitation laser wavelength, suggesting that the vibrational energy partitioning is correlated with the collisional impact parameter. We have also carried out a careful search for the MgCH3 reactive product in this system, finding no evidence for any observable branching to this product. We discuss the implications of these results for the chemical dynamics of this metal-alkane reaction system.

Energy partitioning during the exit channel motion for CH 2OH +→CHO ++H 2

The exit channel dynamics of the four-centered elimination CH 2OH +→CHO ++H 2 is investigated by a classical trajectory calculation on the analytic potential energy surface reported previously (J. Chem. Phys. 104 (1996) 4517). The rotational and relative translational energies of the two separating moieties and the total vibrational energy are calculated as a function of the distance between their centers of mass. Rotational and vibrational energies are found to be determined shortly after the sustem leaves the transition state. Thereafter, the remainder of the reverse barrier is mostly converted to the relative translational energy. Hence, a shift technique may be used to estimate the kinetic energy release distribution for the products from the translational energy distribution obtained from the middle of the exit channel motion.

Probing dynamics in the Franck–Condon and exit channel regions of dissociating H2S: Emission spectra upon tunable excitation from 199–203 nm

This study uses emission spectroscopy of H2S at excitation energies near 200 nm to probe the dissociation dynamics from a conical intersection in the Franck–Condon region to the H+SH product exit channel. Photoexcitation accesses these coupled surfaces near the transition state region of the lower adiabat, a potential surface for the excited state H+SH→HS+H reaction. Excitation wavelengths from 199–203 nm tune through the first of the resonances in the absorption spectrum assigned to recurrences in the motion along the symmetric stretch orthogonal to the reaction coordinate and also access energies just above and at the conical intersection. We disperse the emission from the dissociating molecules at each of five excitation wavelengths in this region to probe several features of the reaction dynamics on the coupled potential energy surfaces. The resulting emission spectra cover the range of final vibrational eigenstates from 500 to 11 000 cm−1 above the initial ground vibrational state for all five excitation wavelengths, and go out to 16 500 cm−1 for the 199 and 201 nm excitation wavelengths. The resulting spectra, when considered in conjunction with recent scattering calculations by Heumann and Schinke on ab initio potential energy surfaces for this system, evidence a progression of emission features to low vibrational eigenstates in the SH stretch that result from coupling of the nuclear motion from the bound to the dissociative region of the potential energy surfaces. This emission, into local mode eigenstates such as 00+1, 11+0, 11+1, 21+0, 21+1, evidences the antisymmetric dissociative motion and bending induced near the conical intersection, and dominates the spectrum at excitation wavelengths only near 200 nm. We analyze the excitation wavelength dependence of these features and also of the n0+0 progression for n≥4, which reflect the exit channel dynamics. The excitation wavelength dependence shows that while the emission spectra do not reveal any dynamics unique to scattering states that access a symmetric stretch resonance in the Franck–Condon region, they do reveal the energy location of and the dynamics at the conical intersection. A reanalysis of other workers’ measurements of the SH product vibrational state distribution shows that v=0 products are strongly favored at excitation wavelengths near the conical intersection.

Wave packet correlation function formulation of scattering theory: The quantum analog of classical S-matrix theory

A novel time-dependent quantum mechanical formulation of scattering theory is developed which is well suited for the calculation of individual S-matrix elements. Wave packets corresponding to well-defined reactant and product channel quantum numbers are propagated in to the interaction region using Mo/ller operators, the former forward in time and the latter backwards in time. The S-matrix element Sβα(E) is then simply related to the Fourier transform at energy E of the time-dependent correlation function between the two wave packets in the interaction region. The symmetric treatment of reactants and products allows the entrance and exit channel dynamics to be performed highly efficiently using different coordinate systems and different interaction representations. As a result, the formulation is expected to provide an improved route to the calculation of S-matrix elements using quantum mechanical, as well as semiclassical propagation methods. The new formulation combines elements of classical S-matrix theory, the wave packet correlation formulation of spectroscopy, and quantum transition state theory, and should be a good starting point for a variety of new approximations to and interpretations of individual S-matrix elements.

Photofragmentation: understanding the influence of potential surfaces and exit-channel dynamics

Photoexcitation is used to prepare species whose subsequent fragmentation can be exploited for the purposes of studying, controlling and manipulating different kinds of molecular processes. First, we show how a form of sub-Doppler resolution spectroscopy can be used to determine centre-of-mass kinetic energy distributions, thereby enabling internal energy distributions to be obtained for elementary processes which occur at a fixed total energy. We present data for H atoms monitored at the Lyman-α wavelength. Secondly, we show how such processes can be used to study binary interactions by preparing van der Waals complexes (e.g. CO2HBr) and then photodissociating one constituent of the complex (HBr +hν→ H + Br), thereby propelling the H atom toward the other constituent with well defined initial conditions. Finally, we turn from such direct fragmentation processes to those in which radiationless decay of S1 leads to reaction via T1 and/or S0 surfaces. For the case of ButNO, we study both channels, and find a barrier of ca. 650 cm–1 on T1. Reaction via S0 is ‘statistical’, while the T1 path leads to dynamical bias. Below the T1 barrier, reaction is slow (>3.5 µs) compared to radiationless decay (<50 ns), thus allowing metastable species to be prepared.